Cellular wireless networks typically comprise wireless devices, including user equipment (UE) such as mobile handsets, etc., which may communicate via a network interface in the wireless device comprising a radio transceiver to a network of base stations connected to a telecommunications network. Such cellular wireless networks have undergone rapid development through a number of generations of radio access technology. The initial deployment of systems using analogue modulation has been superseded by second generation (2G) digital systems such as GSM (Global System for Mobile communications), and these systems have themselves been replaced by or augmented by third generation (3G) digital systems such as UMTS (Universal Mobile Telecommunications System), implementing the UTRAN (Universal Terrestrial Radio Access Network) radio access networks. Third generation standards provide for a greater throughput of data than is provided by second generation systems; this trend is continued with the introduction of Long Term Evolution (LTE) and LTE Advanced systems. Technical specifications for advanced cellular wireless networks are produced by Technical Specification Groups (TSGs) of the 3rd Generation Partnership Project (3GPP).
A trend in the development of advanced cellular wireless networks is the increasing use of distributed antenna systems. For example, advanced technical standards for LTE systems (e.g. LTE Release (Rel)-11 and above) have specifications relating to Multiple Input Multiple Output (MIMO) and coordinated multi-point transmission (CoMP) technologies. Both single-user (SU-) MIMO and multi-user (MU-) MIMO technologies are considered. As well as CoMP technologies there are also options for single or multi-point non-coordinated transmissions. It is envisaged that these technologies will be deployed in both homogeneous and heterogeneous network configurations.
For example, in a case of single point transmission, there may be configurations such as: a homogeneous macro network or a heterogeneous network of macro-sectors with four low power nodes (LPNs) and no coordination. The latter configuration may involve the macro sectors and the LPNs operating on the same or different frequency bands, such as one of the two operating on a higher and/or adjacent frequency band. In the case of multi-point transmission, there may be configurations such as: a homogeneous macro network with intra-site coordination; a homogeneous macro network with high power remote radio heads (RRH); or a heterogeneous network of macro-sectors with four LPNs. In the latter configuration, the LPNs may operate with or without the same cell ID.
A network deployment of a distributed antenna system may thus comprise a plurality of transmission points (TP). Each transmission point has its own antenna configuration. This antenna configuration may vary in terms of, for example, the number and type of antennas that are used. For instance, cross-polarized (XP) antennas or uniform linear arrays (ULA) may be used, with either close (e.g. λ/2—half wavelength) or large (e.g. 4λ—four times wavelength) separation between elements.
In the field of radio access network (RAN) technologies there is thus a challenge to manage distributed antenna systems that may have a wide range of configuration options. For example, a transmission point may comprise multiple antenna ports and there may be a number of geographically-distributed transmission points.
The 3GPP TGS RAN working group dealing with technical specifications for the physical layer (layer 1 in the Open Systems Interconnection (OSI) model), WG1, has discussed antenna port mappings for geographically separated antennas (see, for example, R1-113610, the “Liaison Statement on Antenna Port Mapping onto Geographically Separated Antennas”, 3GPP TSG RAN WG1 Meeting #66bis, Zhuhai, China, Oct. 10-14, 2011). This discussion was prompted by an earlier discussion (R1-111330, “Considerations on Real-Life DL MIMO Aspects”, Ericsson, ST-Ericsson, 3GPP TSG-RAN WG1 #64, Barcelona, Spain, May 9-13, 2011) that considered the accuracy of UE transmission rank reporting, wherein a transmission rank denotes the number of layers that should preferably be used for downlink transmission to the UE, “layers” in this case relating to a transmission coding abstraction that is mapped to one or more antenna ports. In particular implementations, the UE selects or recommends a rank for use which is reported to a base station such as an eNodeB. Transmissions, e.g. from the base station, are then made based on the selected or recommended rank; for example, when rank 1 is reported, a corresponding transmission uses a single spatial layer (or a single stream), that can be mapped onto one or more antennas via spatial precoding and when rank 2 is reported, a corresponding transmission uses two spatial layers (or a dual stream), that can be mapped onto two or more antennas via spatial precoding.
In the earlier discussion it was found that there may be problems for a UE when it receives signals from antenna ports with a large imbalance in received power; for example, rank 2 may be reported even in cases where rank 1 is more efficient. This large imbalance may be due, for example, to geographical separation of the antenna ports. However, the results presented in this earlier discussion may not apply to all implementations. For example, in certain UE implementations, such as the real-world experimental results presented in technical document R1-113178, “Real-life measurements on rank adaptation”, Renesas Mobile Europe Ltd, 3GPP TSG RAN WG1 Meeting #66bis, Zhuhai, China, Oct. 10-14, 2011) it has been shown that transmission rank adaptations could be accommodated in scenarios having large power imbalances.
In the latter discussion on antenna port mappings for geographically separated antennas, it was discussed how UE implementations should not assume geographical co-location for different antenna ports of a given cell, a cell being a spatial area served by a particular base station, or in general dependence among antenna ports. For example, it is assumed that there is flexibility with respect to mapping different antenna ports of a cell to different geographically separated antennas or transmission points. In particular, geographical co-location may not be assumed for, e.g., antenna ports transmitting cell-specific reference signals (CRS), UE-specific demodulation reference signals (DM-RS), and channel-state information (CSI) reference signals (CSI-RS). Deployments with LPNs or RRHs typically assume that the corresponding transmission points are geographically non-co-located. Technical specifications drawn up for the RAN in LTE rely on the independence of different antenna ports for precoding procedures with the mapping from antenna ports to antennas being transparent to the UE.
The issues discussed above were considered in follow-up discussions in the 3GPP TGS RAN working group for radio performance protocol aspects, WG4. In a Liaison Statement in reply to the “Liaison Statement on Antenna Port Mapping onto Geographically Separated Antennas” (see R4-121116, “Liaison Statement on Geographically separated antenna and impact on CSI estimation”, 3GPP TSG-RAN WG4 Meeting #62, Dresden, Germany, Feb. 6-10, 2012), the possibility of introducing new tests to be performed by the UE to verify that no assumption on antenna port co-location is made by the UE was raised. In the reply, the working group expressed concerns about the complexity of evaluating antenna port co-location assumptions for reference signal configurations and required further feedback on the issue. In particular, further investigation was required in at least the following areas: whether any reference signal ports may be assumed as co-located or not; and the most relevant scenarios in terms of antenna ports deployment and power imbalance which need to be considered. The working group also raised the issues of performance degradation and increased UE complexity, which may occur for certain legacy UE implementations that assume arbitrary antenna port co-location.
There are thus numerous outstanding issues for the deployment of a distributed antenna system. In particular, if it can be assumed that antenna ports may be arbitrarily co-located, which is desired for flexible network configurations, there are problems that must be overcome for efficient operation of a wide range of UEs.